X-rays have traditionally been generated by letting an electron beam impact upon a solid anode target. However, thermal effects in the anode limit the performance of the X-ray source. One way of mitigating the problems relating to overheating of the solid anode target has been to use a rotating solid anode.
A second conventional approach is to direct the electron beam towards a liquid anode target, such as in the form of freely falling droplet or stream. U.S. Pat. No. 4,953,191 shows an X-ray source using as its anode a falling stream of liquid gallium, which is substantially flat and typically has a tangential velocity of about 2.0 m/s. In view of overheating, the power of an X-ray source of this type will be limited by the mass of gallium transported per unit time by the flowing stream. Spatial localization is typically a desirable property of the source, and neither thickening the stream nor increasing its speed are available as ways to increase the mass flow.
More recently, it has been proposed to use a liquid jet as electron target in X-ray generation. For example, WO 02/11499 discloses a method and apparatus for generating X-ray or EUV radiation using a liquid jet as a target for an electron beam. Such X-ray sources may comprise a grounded jet of liquid metal adapted to act as anode, i.e., as target for the electron beam. By virtue of its regenerative nature, such jet of liquid metal can withstand strong electron beam impact; as a comparison, the propagation speed of the jet can be similar to or higher than the tangential speed of a conventional rotating anode. Although only a fraction of the energy carried by the electron beam is converted into X-ray energy—which leads to a considerable excess heat generation—these liquid jet X-ray sources are nevertheless characterized by excellent brightness, which brings benefits relating to exposure duration, spatial resolution and new imaging methods, such as phase-contrast imaging.
However, it has been a challenge to devise a liquid metal jet X-ray source that can operate for extended periods of time without interruptions for maintenance. For example, in previous X-ray sources of this kind, an operator has been required to halt the generation of X-rays in order to change or refill a target supply container, or to change or refill containers for pressurized propellant gas that is used for generating the target jet.